Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, Base Transceiver Station (BTS), or AP (Access Point), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for terminals. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
D2D Communication
Recent developments of the 3GPP Long Term Evolution (LTE) facilitate accessing local IP based services in a home, an office, a public hot spot or even outdoor environments. One of the important use cases for a local IP access and local connectivity involves the direct communication between wireless devices such as user equipments in the close proximity of each other, typically less than a few 10 s of meters but sometimes up to a few hundred meters.
This direct mode or D2D enables a number of potential gains over the traditional cellular technique where two devices communicates vi a cellular access point such as e.g. a base station because D2D devices are often much closer to one another than the cellular devices that have to communicate via the cellular access point.
One gain is capacity gain which may comprise reuse gain and hop gain. Regarding the reuse gain, D2D communication may provide reuse of radio resources such as e.g. Orthogonal Frequency Division Multiplex (OFDM) resource blocks, reuse gain. Regarding the hop gain, D2D communication provides a D2D link providing a single hop between the transmitter and receiver points as opposed to the 2-hop link via a cellular access point.
Another gain is maximum achievable rate gain, which may comprise proximity gain. D2D communication may further provide high peak rates due to the proximity and potentially favorable propagation.
A further gain is latency gain. When the UEs communicate over a D2D link, base station forwarding is short cut and the end-to-end latency can decrease.
Random Access Procedure in Cellular Networks
MSG1. A user equipment that has new data to transmit and therefore requires resources sends a first message to the base station. The first message may also be referred to as MSG1, a preamble, or a Random Access Channel (RACH) preamble. MSG is an abbreviation for MeSsaGe. In the first message the user equipment provides an indication to the network about it's resource requirement. The first message comprises a preamble Identity (ID). After the user equipment has transmitted its preamble, it waits for a Random Access Response (RAR) associated with its RA-RNTI to see if the base station heard the preamble. The RA-RNTI comprises the subframe number the user equipment transmitted in +1.
MSG2. When the base station hears the preamble, it responds to the preamble by transmitting a second message, also referred to as MSG2 or RAR, to the user equipment. The second message comprises information about Timing Advanced (TA), Preamble ID and Temporary Cell Radio Network Temporary identifier, (TC-RNTI). TC-RNTI is a number generated by the base station used as the identity of the user equipment within the cell.
MSG3. If the user equipment finds a suitable RAR, then it looks to see if its specific preamble identity is included. If so, then the user equipment assumes it received a positive acknowledge from the base station, and the user equipment uses the TC-RNTI for subsequent signalling to the base station. When the user equipment has assumed that it received a positive acknowledge from the base station, the user equipment sends an RRC connection request (not always included, e.g., when the user equipment is in RRC connected mode, it may just use RACH to request resource grant) and a Buffer Status Report (BSR) Control Element (CE) in a third message, also referred to as MSG3 using resources given by the base station. The Buffer Status Report (BSR) Control Element (CE) indicated the amount of new data to be transmitted. It also sends the identifier such as a Temporary Mobile Subscriber Identity (TMSI), to the base station which is used to resolve the Contention, i.e. if other user equipments uses the same RACH. Please note that the RA-RNTI is determined from the user equipments preamble transmission, the temporary C-RNTI is assigned by the base station.
MSG4. The base station sends a Physical Downlink Control Channel (PDCCH) UL grant or a DL assignment to the user equipment, addressed to the C-RNTI of the user equipment, in a fourth message also referred to as MSG4. In this step, the user equipment, which has received the C-RNTI, continues to set up an RRC connection. However, the RACH may be also used for RRC connected user equipments. If other user equipments used the same RACH, they will try again to send a preamble.
According to a current LTE protocol, in a traditional RACH procedure for cellular user equipments, the user equipment maintains a window also referred to as ResponseWindowSize, for the reception of MSG2, and timer, also referred to as Medium Access Control (MAC)-ContentionResolutionTimer for the reception of MSG4. The possible value of the window is from 2 sub frames to 10 subframes, and the value of the timer is from 8 subframes to 64 subframes. When the window is active, the user equipment would have to keep monitoring Physical Downlink Control Channel (PDCCH) for possible MSG2 on DL. When the timer is active, the user equipment would have to keep monitoring PDCCH for possible MSG4 on DL. If no grant is received within the window or timer, this RACH attempt fails.
In a mixed cellular and D2D network, the addressed RACH procedure is limited to being triggered by new data at the user equipment, i.e., UL data to send to the base station or D2D data to send to the counterpart, where the user equipment is out-of-sync but still RRC connected. Here the out-of-sync means that the UL synchronization to the base station is lost. In the current RACH procedure, receiving a C-RNTI and a BSR CE in UL MSG3, makes the base station aware of the identity of the user equipment that performs the RACH procedure and that it has data to send on UL. The base station then schedules the user equipment on a Physical Uplink Shared CHannel (PUSCH) for UL data transmission, so when the user equipment receives PDCCH UL grant addressed to its C-RNTI, i.e., MSG4, the RACH procedure is seen as successful, and the ContentionResolutionTimer is stopped. However, for a RACH triggered by D2D data, a base station receiving C-RNTI and BSR in MSG3, is blind to whether it is due to cellular or D2D data, and still schedule the user equipment in a cellular UL link, even though, what the user equipment really want is the scheduling of a D2D link.